Under the command of the evil witch Katja, Hansel would perform the spell on adults no one would miss to turn them into teenagers so the witch could eat them as it was harder to kidnap children now without people noticing. After Hansel used the spell on Dean Winchester, he escaped and returned with his brother Sam. They forced Hansel to tell them how to reverse the spell and then tried to kill the witch with Bobby's witch-killing spell. However, Hansel was evil and stopped them. To save Sam and stop Katja, Dean managed to steal Hansel's hex bag and reverse the spell. An adult again, Dean then kills Hansel and Katja, but as the hex bag was burned in the fight, they are unable to reverse the spell on Tina who sees it as a second chance.
The spell was performed by the caster squeezing a hex bag full of unknown ingredients and yarrow powder, with a pentagram with a circle around it on it. The spell caused a flash of white light and the person the spell was used on disappeared, leaving behind their clothes and yarrow. They were then transported to a cell in the witch's house as a teenager.
Reversing the spell was simple: the person affected simply had to squeeze the hex bag themselves and they would return to their proper age. However, if the hex bag is destroyed, the spell can't be reversed.
Interestingly, the spell seems to also influence time itself in relevance to the recipient and not merely a case of magically-induced alteration of physical form. This was best demonstrated when the spell affected Dean; not only did it revert him back into a teen, but also nullified the Mark of Cain as he was sent back to the age where he has yet to acquire it.